1. Field of the Invention
The present invention relates to a lithium secondary battery having a high efficiency charging lifespan characteristic and enabling high capacity charging in a short time and, more particularly, to a lithium secondary battery including a water-dispersible binder, a conduction agent, and fluoroethylenecarbonate.
2. Discussion of the Related Art
Today, commercial electrode binders chiefly used to fabricate secondary batteries include polyvinyllidene fluoride (hereinafter referred to as ‘PVDF’)-based polymers, PVDF homopolymers, polyvinylidene fluoride hexafluoropropylene copolymers (Korean Patent Application Publication No. 2001-0055968), and polyvinyllidene flouride-chlotrifluoroethylene copolymers.
The PVDF-based polymers are advantageous that they are stable chemically and electrochemically, but may have environmental problems resulting from organic solvents, such as NMP (N-methyl-2-pyrrolidone), because they have to be dissolved in the organic solvents and used as binder compositions.
Further, the PVDF-based polymers are dangerous because of a low safety and become the root cause of a reduction in the performance of electrodes due to a low affinity with a liquid electrolyte.
In addition, the PVDF-based polymers are excellent in the binding characteristic with inorganic substance particles, such as active materials, because they act with it surrounding the circumference of the active materials, but are disadvantageous it that they must be used in a large quantity in order to exhibit and maintain sufficient adhesive strength because they have poor adhesive strength with a current collector such as metal.
In order to solve the above problems, a water-dispersible electrode composition using water as a dispersion medium (i.e., a solvent) was proposed. In this case, a water-dispersible binder is used instead of the above-described PVDF-based binder. Styrenebutadiene rubber (SBR) is chiefly used as the water-dispersible binder. In an electrode using the water-dispersible binder, a binding effect is higher than that of a non-aqueous (i.e., an organic solvent-based) binder although the water-dispersible binder is used in a small quantity and thus the ratio of presence of active materials per the same volume can be increased, thereby being capable of achieving a high capacity and a long lifespan characteristic. Accordingly, it is expected that batteries adopting negative electrodes using the water-dispersible binder will become the main stream in the future.
For a conventional process of fabricating the negative electrodes of a secondary battery using water as a dispersion medium, reference can be made to a document 1 below. The document 1 discloses influence of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) on the stability of suspensions in a process of fabricating water-based suspensions of natural graphite which is a material for a lithium ion battery negative electrode; electrokinetic behavior and flexible behavior in order to evaluate the dispersion stability of suspensions according to organic additives; a shaping micro structure and a pore ratio of as-castsheet and correlation therebetween, etc.
[Document 1] Jin-Hyon Lee, ┌Process of fabricating water-based suspensions of materials for negative electrodes of lithium ion batteries and evaluation of battery characteristics┘, a thesis for a master's degree, Hanyang University, 2005.